This is my first post on here but Im after some clarification.
The sitiuation is a house on PME supply, rewired in 80's but CU replaced with 17th Ed a few years ago, with the preposal to take a feed to a largish metal garage at the end of the garden, on concrete slab, no water/gas or other earth path at present, around 10meters from the house.

I understand that it is possible to export the earth to a garage, as long at the CPC is over 10mm^2 csa back to the MET and tests out ok, and that using 3core 6mm^2 SWA the armour and one of the cores together will give you the equivelent of 10mm^2 copper. I have also seen 10mm^2 G/Y pinned along side SWA to provide the earth, but while I would be happy with this inside im not overly happy with it secured along the garden wall exposed to all eliments and not mechanically protected.

However the preposal is to reappropreate and extend an unused cooker feed (6mm^2 T&E) which on the back wall of the house, rather than run the SWA back to the consumer unit at the front of the house either through the house or outside it, to reduce disterbance to the house or the unsightlyness of having SWA run round three walls and over the front door.
This appears a valid suggestion, other than the fact the earth at the cooker point is obviously only 4mm^2 as part of the T&E. It would be possable to extend the CPC that bonds to the gas pipe to the MET (also at the back of the house, but about 4m away and the other side of a door) or take a new cable back to the MET, but neather is also not a trivial task.

The other option as I see it is therefore to install an earth rod at the garage and have the garage on a TT earth system. At which point you can use 2core 6mm^2 SWA onto the end of the existing 'cooker feed' earthing the armouring at the supply end, then at the garage, terminate the SWA without earthing the armouring. With the earth being taken from the proposed earth rod, which will also be bonded to the garage.

All circuits in the garage will be RCD protected by a consumer unit in the garage, preposed as 6amp lighting circuit, and two 16amp radials of sockets.
The cooker feed would be left in the house CU protected by a 40amp MCB and one of the two RCD's, due to the basic nature of the rest of the house wiring and the 17th ed board, the risk of a fault in the garage tripping the RCD I feel is low enough to justify not taking a non RCD protected feed out before the CU and through a dedicated switch fuse. And you atleast should acheive discrimination against the 40amp MCB in case of an overload fault and maintain the lights in the garage.

Is this a suitable solution. One of my main concerns is how to terminate the SWA at the garage without earthing it, the preposed garage CU like many is metal, I have in the past seen SWA terminated into a plastic box, running the L+N conductors on into consumer unit, but even then the SWA gland is earthed and only sleaved with the boot. Another suggestion was to terminate the SWA in a plastic/rubber gland (which could then be into the metal cu?) and trim back the armouring and insulate with tape, but im not overly happy about this either.

Personally, I'd avoid using PME for a metal-clad outbuilding. There's not just the risk of dangerous shocks in the case of a broken supply CNE (caravan style), but of perceived (lower voltage) shocks in normal circumstances too - which can do a lot of damage to an electrician's reputation even if it's not physically dangerous. Garage doors especially are prone to being grabbed hold of by people in all weathers.

Using a spare core/armour of SWA as a PME bonding conductor also raised the problem of the heat generated by diverted N currents flowing to earth through extraneous-conductive-parts in effect de-rating the cable (as GN 8 mentions) - by how much is more of an art than a science....

As for terminating the SWA in a TT system - a plastic stuffing gland in a plastic enclosure for the CU is probably easiest.

To stick to the letter of the regs I think you'll need an RCD in the garage. I know the one in the house will give adequate protection in practice, but the regs require all exposed-conductive-parts protected by a given device to be connected to the same earthing system (411.5.1 and 411.4.2), which obviously won't be the case if the garage is TT'd. A plastic CU avoids the problem of faults to earth of the supply tails before the garage's RCD(s). If you're committed to a metal CU, a plastic modular enclosure containing the RCD before it and with SWA terminating into that would be common solution.

It would be much neater to take the SWA back to the house CU and omit RCD protection at that end. Are the ground floors suspended? (i.e. could it go under?) - or trench around the house? or go up to ground floor ceiling/1st floor void level in some inconspicuous corner and through the floor void to the CU?

I understand that it is possible to export the earth to a garage, as long at the CPC is over 10mm^2 csa back to the MET and tests out ok, and that using 3core 6mm^2 SWA the armour and one of the cores together will give you the equivelent of 10mm^2 copper.

You can't make that assumption - the requiremenmt is for 10mm2 of copper or material of equivalent conductance.

You have 6.0mm2 of copper - the steel wire armour on a 3c x 6.0mm is about 23mm2. The conductivity ratio of steel to copper is about 9 - so you only have 23/9 = 2.5mm2 copper equivalence - in total that gives you 6 + 2.5 = 8.5mm2 - so not big enough.

In practice, it's easier to put in a 10.0mm2 3 core and not to make a demand on the armour (although you would still earth it)

Im not committed to a metal CU in the garage, there just fairly commonly metal and I have a spare one! Not the end of the word to pick up an 8 way plastic wylex.

I would love to take it back to a switched fused, or even pre rcd mcb but there is no isolation between the servicehead and the CU and the house CU doesnt have any pre-rcd ways (also a wylex box) and as far as I know short of swapping the whole box for one that does have some pre RCD ways you cant adapt them?
At which point given it can almost be on its own RCD the risk seems small. The house only has one ring main, up and down stairs lighting, a lolipop to the convervatory, the unused cooker feed, and an unused emersion feed so its half empty.

As for getting a cable there, the floor in the kitchen and bathroom (accross the back wall) is concreate so you cant go under, there a false plasterboard ceiling in the kitchen, artex in the hall way, and structural walls either side. Obviously anything is possable, but again, given the 6mm already running to the back wall the risk/benifit/cost/effort balance of it all seems not to swing in its favor, and equally I can see that 6mm SWA running over the conservatory, round the kitchen, bathroom, along the front wall, and in to the CU, appears a a bit industrial and the sort of thing you might find the council do, its that sort of area!

OMS, I've not done the sums myself, but its something I hear banded around a fair bit. Looks like a localised TT is prefered anyway.